Replaced build system (GNU Autotools -> CMake)

Replace item identifiers with message digests

   A feed, found in the wild, highlighted a software defect: multiple feed
   entry having the same entry IDs could not be individually handled.

   Entries will be no longer uniquely identified by the GUID or Link,
   rather by the SHA-1 digest of the concatenation of a few entry fields.
   The current selection of fields are Title, GUID, Link and Publishing Date.

   To obtain the digest we rely on libcrypto.

Dropped crossbow-upgrade(1).

Various minor bugfixes and code and testing improvements.